On 29 August San Cristóbal erupted briefly with a minor ejection of ash -- the eruption being another signal in the rise of the volcano's thermal gradient as evidenced by earlier eruptions in May 1971 and 9-10 March 1976, both of which were minor. The volcano has been quiet since the most recent eruption.

According to Richard Stoiber, both gas emission and its SO₂ content increased from early 1971 to March 1976, but had declined by mid-July 1976. The ash ejection of 29 August has increased anticipation of major activity. Stoiber has been monitoring the volcano for the past 6 months, collecting condensed water samples from active fumaroles. Analysis of those samples has not yet been completed.

Geological Summary. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: A. Aburto Q., Instituto de Investigaciones Sísmicas.